gpu: drm: amdgpu: Replace mdelay with msleep in cik_pcie_gen3_enable()

cik_pcie_gen3_enable() is only called by cik_common_hw_init(), which is
never called in atomic context.
cik_pcie_gen3_enable() calls mdelay() to busily wait, which is not
necessary.
mdelay() can be replaced with msleep().

This is found by a static analysis tool named DCNS written by myself.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>